Production method of cellulose film, cellulose film, protective film for polarizing plate, optical functional film, polarizing plate, and liquid crystal display

ABSTRACT

A production method of cellulose film wherein cellulose film is produced by preparing a polymer solution through dissolving cellulose ester in a solvent containing a prescribed organic solvent as the main component, forming a filmy object from the prepared polymer solution, and evaporating the solvent in the filmy object; the residual amount of the organic solvent is reduced while the film quality is not degraded, and the production efficiency is degraded to a least possible extent; a poor solvent, highest in boiling point among the materials contained in the solvent, is added in the content ranging from 0.1 wt % to 1.0 wt %, taking the total amount of the solvent in the prepared polymer solution to be 100 wt %; and the solubility of cellulose ester in the poor solvent is inferior to the solubility of the cellulose ester in the organic solvent which is the main component of the solvent.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a production method of cellulosefilm in which cellulose film is produced by preparing a polymer solutionthrough dissolving cellulose ester in a solvent containing a prescribedorganic solvent as the main component, forming a filmy object from theprepared polymer solution, and evaporating the solvent in the filmyobject, and a cellulose film thus produced; and protective film forpolarizing plate, optical functional film, polarizing plate, and liquidcrystal displays produced by the above described production method ofcellulose film.

[0003] 2. Description of the Related Art

[0004] Conventionally, cellulose film is used as optical materials foroptical functional films for use in widening the viewing angle andpreventing glare; protective films for the polarizing plates in liquidcrystal displays; and the like. The cellulose films used for suchoptical materials are produced by means of the solution method forforming film. In the solution method for forming film, a filmy object isformed from a polymer solution in which cellulose ester or the like isdissolved in an organic solvent, the filmy object formed is heated toevaporate the organic solvent in the filmy object, and a polymer film isthereby obtained. In this connection, when the organic solvent remainsin the produced cellulose film, there occur adverse effects on thedimensional stability of the film, or the coloring of the film isdegraded. Accordingly, the control of the residual amount of the organicsolvent in the produced cellulose film has heretofore been performedfrom the viewpoint of the quality. The produced cellulose film issubjected to saponification and the like in post-processes, andsubsequently is commercialized as optical functional film, or protectivefilm for polarizing plate.

[0005] Now, when the cellulose film produced by the solution method forforming film is considered from the standpoint of the environmentconservation, nowadays considered to be important, there is concernthat, with the level of the residual amount of the organic solvent ascontrolled from the viewpoint of the quality, a slight amount of theorganic solvent is evaporated from the produced film in thepost-processes subsequent to the film production.

[0006] However, as for the residual amount of the organic solvent,investigation has hitherto been performed from the viewpoint of thequality, but no investigation has been performed from the viewpoint ofthe environment conservation, and hence it is not clear how far thelevel of the residual amount of the organic solvent should be lowered sothat the effects on the environment substantially vanish.

[0007] Additionally, in order to reduce the residual amount of theorganic solvent, the following treatments are suggested: the heatingperiod of time is extended in the evaporation process, the heatingtemperature is raised, and the amount of the organic solvent is reducedin relation to the amount of cellulose ester. However, in the currentsolution method for forming film, the production efficiency is improvedwhile the quality of the film being maintained at a high level, andhence a variety of measures are adopted for shortening the time requiredfor the evaporation process as much as possible; these measures includethe following measures in which the amount of the organic solvent isdecreased to a level as low as possible in relation to the amount ofcellulose ester, and the heating temperature is raised to a level belowwhich cellulose ester is not thermally decomposed. Consequently, it isanticipated that not only the quality of the film is degraded, but alsothe production efficiency is remarkably degraded, owing to extending ofthe heating time in the evaporation process, raising the heatingtemperature, and reducing the amount of the organic solvent in relationto the amount of cellulose ester, for the purpose of reducing theresidual amount of the organic solvent.

SUMMARY OF THE INVENTION

[0008] The present invention, in view of the above circumstances, takesas its object the provision of a production method of cellulose filmwhich can reduce the residual amount of the organic solvent, withoutdegrading the film quality, and with degrading the production efficiencyto the least possible extent; a cellulose film which substantially haslittle effects on the environment due to the residual organic solvent;and protective film for polarizing plate, optical functional film,polarizing plate, and a liquid crystal display produced by the abovedescribed production method of cellulose film.

[0009] The production method of cellulose film of the present invention,which achieves the above described object, is a production method ofcellulose film which method produces cellulose film by preparing apolymer solution through dissolving cellulose ester in a solvent havinga prescribed organic solvent as the main component, forming a filmyobject from the prepared polymer solution, and evaporating the solventin the filmy object; and wherein:

[0010] the polymer solution is prepared by adding a poor solvent, havingthe highest boiling point among the materials contained in the solvent,so as to have the content of 0.1 to 1.0 wt % where the total amount ofthe solvent in the prepared polymer solution is taken as 100 wt %; and

[0011] the solubility of cellulose ester in the poor solvent is inferiorto the solubility of cellulose ester in the organic solvent which is themain component of the solvent.

[0012] The addition amount of the poor solvent is very small, so thatthe addition of the poor solvent scarcely degrades the productionefficiency of the cellulose film. Additionally, since the poor solventis highest in boiling point among the materials contained in thesolvent, it is most difficult to be evaporated and tends to remain.Furthermore, since the solubility of cellulose ester in the poor solventis inferior to the solubility of cellulose ester in the organic solventwhich is the main component of the solvent, the intermolecular bondbetween the poor solvent and the cellulose ester is difficult to beformed as compared to the intermolecular bond between the main-componentorganic solvent and the cellulose ester. In the solvent during theevaporation process, the action of the remaining poor solvent preventsthe formation of the intermolecular bond between the main-componentorganic solvent and cellulose ester, and the evaporation of themain-component organic solvent is thereby promoted. Additionally, theremaining poor solvent hardly forms the intermolecular bond withcellulose ester so that cellulose ester is scarcely restrained bycellulose ester, and the addition amount of the poor solvent is verysmall; hence the poor solvent is evaporated at the end of theevaporation process, and the added poor solvent does not affect the filmcharacteristics.

[0013] Additionally, in the production method of cellulose film of thepresent invention, the main-component organic solvent isdichloromethane, and it is preferable that the polymer solution isprepared by adding an alcohol having one to two carbon atoms in additionto the poor solvent.

[0014] The compatibility of dichloromethane with cellulose ester issatisfactory, and hence adoption of dichloromethane as the maincomponent of the solvent leads to reduction of the total amount of thesolvent in relation to the amount of cellulose ester. Additionally,addition of alcohols having 1 to 2 carbon atoms improves the dimensionalstability (self-supporting property) of the filmy object, making thetransportation of the film-like material be convenient.

[0015] Furthermore, in the production method of cellulose film of thepresent invention, taking the total amount of the solvent in theprepared polymer solution to be 100 wt %, it is preferable to preparethe polymer solution in such a way that dichloromethane is added in acontent of 70 to 99 wt %, and simultaneously an alcohol having 1 to 2carbon atoms is added in a content of 0.9 to 29.0 wt %.

[0016] Additionally, in the production method of cellulose film of thepresent invention, it is preferable that the poor solvent is an alcoholhaving the boiling point in the range from 80 to 170° C.

[0017] The boiling point of dichloromethane, the main solvent componentof the solvent, is about 40° C.; accordingly, when the boiling point ofthe added alcohol is 80° C. or above, the alcohol remains in the solventduring the evaporation process, preventing without fail theintermolecular bonding formation of dichloromethane with celluloseester. On the other hand, when the boiling point of the added alcohol ischosen to be 170° C. or below, the alcohol can be evaporated in thefinal stage of the evaporation process without causing the thermaldecomposition of cellulose ester.

[0018] In this connection, in the production method of cellulose film ofthe present invention, when the mixing of the poor solvent is performedin an in-line mode, a static mixer may be used in the piping foraddition and mixing; or at least two or more kinds of polymer solutionsmay be subjected to simultaneous flow casting or successive flowcasting.

[0019] Additionally, in the production method of cellulose film of thepresent invention, it is also preferable that the polymer solution ofcellulose ester film has the solid content ranging from 15 to 30 wt %.

[0020] Additionally, in the production method of cellulose film of thepresent invention, it is also preferable that the material containingthe cellulose acetate synthesized from wood pulp as the main componentis used as cellulose ester.

[0021] As cellulose ester, the cellulose acetate synthesized from cottonlinter is known, in addition to the cellulose acetate synthesized fromwood pulp; however, adoption of the cellulose acetate synthesized fromwood pulp as the main component makes it possible to reduce the costsfor cellulose film.

[0022] Additionally, in the production method of cellulose film of thepresent invention, it is preferable that the film is made to be swollenand then dried on the way of the drying process thereof, or afterdrying, during the film formation process by flow casting of the polymersolution of cellulose ester.

[0023] As above, through swelling once the filmy object, while thesolvent being evaporated from the filmy object, or after the solvent hasbeen evaporated, even when the molecules composing the solvent form theintermolecular bond with cellulose ester, the intermolecular bond can bebroken; namely, the evaporation of the solvent can be further promotedby swelling once the filmy object and then evaporating the solventtherein again.

[0024] In this connection, it is preferable that for the purpose ofswelling once the film (filmy object), the film may be swollen withwater, a solvent may be applied onto the film, or exposure to a solventgas may be performed. Incidentally, it is preferable that analcohol-based substance (for example, an alcohol having 1 to 2 carbonatoms, etc.) is used as the solvent to be applied and the solvent gas.

[0025] The cellulose film of the present invention, which achieves theobject of the present invention, is characterized in that, in the formof the finished film product, the residual amount of dichloromethane is0.1 wt % or less, and additionally the total residual amount of thesolvent is 0.5 wt % or less.

[0026] By controlling the residual amount of dichloromethane, and thetotal residual amount of the solvent to the values as specified above,the effects on the environment of the residual solvent in the cellulosefilm having been produced can be substantially prevented.

[0027] The protective film for polarizing plate, optical functionalfilm, polarizing plate, and liquid crystal displays, which achieve theobject of the present invention, are characterized in that each thereofis produced by use of the production method of cellulose film of thepresent invention, or by use of the cellulose film of the presentinvention.

[0028] As above, the present invention can provide the production methodof cellulose film in which the residual amount of the organic solvent inthe film can be reduced, without degrading the film quality and withdegrading the production efficiency to a least possible extent; thecellulose film which gives the substantially vanishing effects of theresidual solvent on the environment; and the protective film forpolarizing plate, optical functional film, polarizing plate, and aliquid crystal display, all produced by the aforementioned productionmethod of cellulose film.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a simplified schematic view of the production line whilethe cellulose film is being produced by flow casting of the polymersolution onto a round cylindrical drum; and

[0030]FIG. 2 is a simplified schematic view of the production line whilethe cellulose film is being produced by flow casting of the polymersolution onto an endless belt.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Description will be made below on the embodiments of the presentinvention.

[0032] In the embodiments of the production method of cellulose film ofthe present invention, the cellulose film is produced by flow casting ofa polymer solution onto a supporting body. The supporting bodies ontowhich the flow casting of the polymer solution is performed include thefollowing two bodies, a round cylindrical drum and an endless belt. Now,with reference to FIG. 1, description is made on the production linewherein a cellulose film is produced by the flow casting of the polymersolution onto the round cylindrical drum.

[0033]FIG. 1 is a simplified schematic view of the production line whilethe cellulose film is being produced by flow casting of the polymersolution onto a round cylindrical drum.

[0034] The production line 1 shown in FIG. 1 is one embodiment of theproduction method of cellulose film of the present invention, namely, aproduction line of TAC (triacetyl cellulose) film; from the upstreamside of the production line 1 in order, there are arranged a polymersolution preparation apparatus 100, a flow casting die 200, a flowcasting drum 300, and a peeling roll 400.

[0035] In the production line 1, there are arranged three polymersolution preparation apparatuses 100, each preparing a different type ofpolymer solution. Here, as an example, description will be made on thepolymer solution preparation apparatus 100 shown on the right in FIG. 1.The polymer solution preparation apparatus 100 prepares the polymersolution of triacetyl cellulose. In preparation of the polymer solution,triacetyl cellulose is dissolved in a solvent containing dichloromethaneas the main component, and a small amount of n-butanol is added to thesolution in which triacetyl cellulose is dissolved. n-Butanol is higherin boiling point than dichloromethane. Additionally, the solubility oftriacetyl cellulose in n-butanol is inferior to the solubility oftriacetyl cellulose in dichloromethane. The polymer solution preparationapparatus 100 comprises a storage tank 110, a liquid delivery pump 120,a poor solvent supply device 130, a static mixer 140, and a filter 150.In the storage tank 110, a mixed solution CM of dichloromethane andmethanol wherein triacetyl cellulose is dissolved is stored while beingstirred by stirring blades 111. The solution CM stored in the storagetank 110 is delivered to a flow casting die 200 by the liquid deliverypump 120; the static mixer 140 and the filter 150 are arranged in somemidway points along the liquid delivery path. Additionally, the poorsolvent supply device 130 supplies n-butanol B to the static mixer 140under favor of a liquid delivery pump 131. In the static mixer 140,n-butanol B is added to and mixed with the mixed solution CM. Theaddition amount of n-butanol is so small that the adding and mixing ofn-butanol little degrades the production efficiency of the TAC film.Incidentally, instead of using the poor solvent supply device 130,n-butanol may be added to and mixed in the storage tank 110. The filter150 removes, from the solution delivered from the static mixer 140,foreign objects, undissolved raw materials, etc., and then the solutionis delivered to a flow casting die 200.

[0036] In the flow casting die 200, the polymer solutions preparedrespectively in the three polymer solution preparation apparatuses 100are supplied. More specifically, from the polymer solution preparationapparatus 100 depicted at the right of FIG. 1, of the three polymersolution preparation apparatuses, the polymer solution constituting thefilm surface layer is delivered; from the polymer solution preparationapparatus 100 depicted at the center, the polymer solution constitutingthe film central part is delivered; and from the polymer solutionpreparation apparatus 100 depicted on the left, the polymer solutionconstituting the film back surface layer is delivered. The respectivedelivered polymer solutions are discharged from the outlet of the flowcasting die 200. Incidentally, the number of the polymer solutionpreparation apparatuses 100 is not necessarily limited to 3, but it maybe one, two, or more than three depending on the specification of theproduced cellulose film.

[0037] The flow casting drum 300 is revolved along the direction of thearrow A. The flow casting die 200 is arranged above the flow castingdrum 300 in such a way that the outlet faces onto the circumferentialsurface of the flow casting drum 300.

[0038] The respective polymer solutions discharged from the outlet ofthe flow casting die 200 are subjected to simultaneous flow casting ontothe circumferential surface of the flow casting drum 300 rotating alongthe direction of the arrow A. The polymer solutions discharged onto thecircumferential surface of the flow casting drum 300, during aboutthree-quarter revolution along the direction of the arrow A, iswater-cooled from the inside of the flow casting drum 300 andsimultaneously air-cooled from the outside by blasting cooling air, andaccordingly the gelation is promoted to form a filmy object havingself-supporting property. Subsequently, the filmy object reaches theposition, where a peeling roll 400 is installed, to be peeled off.

[0039] A winding device 500 is arranged at the downstream end of theproduction line 1 shown in FIG. 1. A soft film drying zone 11 and a latestage drying zone 12 are arranged between the peeling roll 400 and thewinding roll 500, both zones being the zones where the solvent in thefilmy object is evaporated. Furthermore, a swelling device 600 isarranged between the soft film drying zone 11 and the late stage dryingzone 12. The filmy object peeled off by the peeling roll 400 isdelivered by two driving rolls along the direction of the arrow B, viathe soft film drying zone 11→the swelling device 600→the late stagedrying zone 12, and wound by the winding roll 500.

[0040] A tenter 700 is arranged in the soft film drying zone 11. Thefilmy object peeled off by the peeling roll 400 is delivered to the softfilm drying zone 11, and passes through the interior of the tenter 700.Inside the tenter 700, the filmy object is heated, the solvent containedin the filmy object is further evaporated. The filmy object dischargedfrom the tenter 700 is delivered to the swelling device 600. A swellingdevice is a device in which the filmy object delivered thereto is oncemade to be swollen; the swelling device 600 in FIG. 1 is a device wherethe solvent gas composed of ethanol is sprayed onto the filmy objectbeing delivered. Incidentally, instead of ethanol, alcohols such asmethanol and water vapor may be sprayed. The filmy object dischargedfrom the swelling device 600 is delivered to the late stage drying zone12. Plural rolls 800 are arranged in the late stage drying zone 12, andthe filmy object is delivered while being wrapped around the pluralrolls 800. The filmy object is heated in the upstream section 12 a ofthe late stage drying device 12, and the solvent remaining in the filmyobject is evaporated.

[0041] As for the n-butanol added to and mixed in the polymer solutionpreparation apparatus 100, it is most resistant to evaporation and tendsto remain in the solvent, since n-butanol is highest in boiling pointamong the materials contained in the solvent. Additionally, since thesolubility of triacetyl cellulose in n-butanol is inferior to thesolubility of triacetyl cellulose in dichloromethane, it is moredifficult to form the intermolecular bond of n-butanol with triacetylcellulose than the intermolecular bond of dichloromethane with triacetylcellulose. In the solvent having been added and mixed with n-butanol,the action of n-butanol breaks the intermolecular bond betweendichloromethane and triacetyl cellulose, promoting the evaporation ofdichloromethane. Additionally, since the remaining n-butanol does nottend to form intermolecular bond with triacetyl cellulose, it does nottend to be engaged to triacetyl cellulose, and its addition amount issmall, it is evaporated by the time when the filmy object has passed theupstream section 12 a of the late stage drying zone 12, so that theadded n-butanol does not affect adversely the characteristics of thefilm. Furthermore, since in the production line 1, the solvent in thefilmy object is once evaporated in the soft film drying zone 11 and issubsequently swollen by the swelling device 600, even the intermolecularbond between dichloromethane and triacetyl cellulose which remainsunbroken by n-butanol can be broken. Then, in the upstream section 12 aof the late stage drying zone 12, the solvents remaining in the filmyobject, such as the dichloromethane broken out of triacetyl cellulose bythe swelling action, is evaporated. Consequently, in the production line1, the evaporation of the solvent in the filmy object is promoted, andthe remaining amount of dichloromethane in the filmy object havingpassed the delivery zone 12 can be reduced to be 0.1 wt % or less, andsimultaneously the total residual amount of the solvent is also reducedto be 0.5 wt % or less.

[0042] In the downstream section 12 b of the late stage drying zone 12,the filmy object is cooled down to room temperature, and the filmyobject (TAC film) takes the form of the finished TAC film product. Thefilmy object (TAC film) discharged from the late stage drying zone 12 iswound by the winding device 500. The TAC film thus produced issubsequently delivered to the subsequent processes, unshown in thefigure, and is commercialized as the optical functional films such asprotective film for polarizing plate and anti-glare film. Additionally,polarizing plate is formed by attaching the protective film forpolarizing plate onto both sides of a polarization element made ofpolyvinyl alcohol etc.; and a part of a liquid crystal display is madeby using the polarizing plate.

[0043] Now, with reference to FIG. 2, description will be made below onthe production line wherein cellulose film is produced by flow castingof the polymer solution onto an endless belt.

[0044]FIG. 2 is a simplified schematic view of the production line whilethe cellulose film is being produced by flow casting of the polymersolution onto an endless belt.

[0045] The production line 2 shown in FIG. 2 is the TAC (triacetylcellulose) film production line which is an embodiment of the productionmethod of cellulose film of the present invention, as the productionline 1 shown in FIG. 1, wherein a flow casting band 900 is arrangedinstead of the flow casting drum 300 arranged in the production line 1shown in FIG. 1. With the same reference numerals for the sameconstituent elements as those of the production line 1 in FIG. 1,description is made below with a focus on the points different fromthose in the production line 1 shown in FIG. 1.

[0046] In the production line 2 shown in FIG. 2, the three same polymersolution preparation apparatuses 100 as the three polymer solutionpreparation apparatuses shown in FIG. 1, and three flow casting dies 200are arranged. The three polymer solution preparation apparatuses 100 arerespectively connected to the three flow casting dies 200 in aone-to-one relation. Additionally, the three flow casting dies 200, flowcasting band 900, and peeling roll 400 are arranged in a drying chamber10.

[0047] The flow casting band 900 is formed by wrapping an endless belt930 around a driving drum 910 and a driven drum 920. The belt 930displaces circularly along the direction of the arrow C in the dryingchamber 10. The three flow casting dies 200 are arranged along therunning direction of the belt 930 and above the belt 930, with the dieoutlets facing onto the surface of the belt 930.

[0048] The polymer solutions delivered to the respective flow castingdies 200 are successively subjected to flow casting onto the surface ofthe belt 930 circularly running along the direction of the arrow C, thesolvent is gradually evaporated while the belt 930 is circularly runningin the drying chamber 10, and becomes a film to yield theself-supporting property. Namely, the evaporation of the solvent leadsto a filmy object having shape stability. After the belt 930 hasfinished about one round, the filmy object is peeled off by the peelingroll 400, and delivered to the soft film drying zone 11.

[0049] In the soft film delivery zone 11 of the production line 2 shownin FIG. 2, plural rolls are arranged; the filmy object going into thesoft film drying zone 11 is delivered along the direction of the arrowD, by being guided by the plural rolls. The swelling device 600 isarranged in a midway position in the soft film delivery zone 11.Incidentally, the swelling device 600 may be arranged in a midwayposition in the upstream section 12 a of the late stage drying zone 12.The swelling device 600 shown in FIG. 2 is different from the swellingdevice shown in FIG. 1 in that the filmy object being delivered iswatersoaked and rinsed with water. Incidentally, the application of analcohol such as ethanol may replace the watersoaking. Both in theupstream section and in the downstream section of the swelling device600 of the soft film drying zone 11, dry air is blasted onto the filmyobject being delivered, resulting in evaporation of the solvent in thefilmy object. In the upstream section 12 a of the late stage drying zone12, next to the soft film delivery zone 11, the filmy object is heated,and the solvent remaining in the filmy object is evaporated.Additionally, in the downstream section 12 b of the late stage dryingzone 12, the filmy object is cooled down nearly to room temperature, totake a form of the finished TAC film product. The filmy object (TACfilm) discharged from the late stage drying zone 12 is wound by thewinding device 500.

[0050] Now, detailed description is made below on the preparation of thepolymer solution. In the polymer solution preparation performed in thepolymer solution preparation apparatuses 100 as shown in FIGS. 1 and 2,at the beginning, triacetyl cellulose grains are dissolved in an organicsolvent having dichloromethane as the main component, in the storagetank 110. The triacetyl cellulose is a mixture of those synthesized fromwood pulp and cotton linter, wherein the content of that synthesizedfrom wood pulp is 60 wt % and the rest of 40 wt % is allotted to thatsynthesized from cotton linter. As above, making that synthesized fromwood pulp be the main component can reduce the cost for the TAC film.Incidentally, that synthesized from cotton linter may be completelyexcluded to make the whole comprise only that synthesized from woodpulp. The compatibility between the dichloromethane and triacetylcellulose is satisfactory, and hence adopting dichloromethane as themain component of the organic solvent leads to the reduction of thetotal amount of the solvent in relation to the amount of triacetylcellulose. Additionally, the organic solvent in the storage tank 110contains methanol as a component of the mixed solvent. The addition ofmethanol leads to the improvement of the shape stability(self-supporting property) of the filmy object peeled off by the peelingroll 400, and the easiness in transporting the filmy object. Thecomposition ratio between the dichloromethane and methanol is soadjusted in the storage tank 110 that dichloromethane is contained inthe content of from 70 wt % to 99 wt %, and methanol is contained in thecontent of from 0.9 wt % to 29.0 wt %, taking the total amount of thesolvent in the polymer solution prepared in the polymer solutionpreparation apparatus 100 to be 100 wt %. Incidentally, ethanol mayreplace methanol, or water may be added with modified composition ratioof methanol. Furthermore, in the organic solvent in the storage tank110, a plasticizer, an ultra violet light absorber, ananti-deterioration agent, etc. are dissolved as additives. In thestorage tank 110, the solid content such as triacetyl cellulose and theadditives is adjusted so as to be from 15 to 30 wt %, taking the amountof the polymer solution prepared in the polymer solution preparationapparatus 100 to be 100 wt %.

[0051] n-Butanol, a poor solvent, is so added that the content thereoffalls in the range from 0.1 wt % to 1.0 wt %, taking the amount of thepolymer solution prepared in the polymer solution preparation apparatus100 to be 100 wt %. Incidentally, as a poor solvent, any alcohol havingthe boiling point in the range from 80 to 170°, other than n-butanol,may be used. The boiling point of dichloromethane is about 40° C.;accordingly, when the boiling point of the poor solvent is 80° C. orhigher, the poor solvent remains in the solvent during evaporation ofthe solvent, and the intermolecular bonding of dichloromethane totriacetyly cellulose is prevented without fail. On the other hand, whenthe boiling point of the poor solvent is 170° C. or lower, the poorsolvent can be evaporated without thermally decomposing triacetylcellulose.

[0052] As a result of the preparation described above, the solvent ofthe polymer solution delivered to the flow casting die 200 is composedof dichloromethane and n-butanol. Additionally, the composition ratiosthereof are such that the content of dichloromethane ranges from 70 wt %to 99 wt %, the content of methanol ranges from 0.9 wt % to 29.0 wt %,and the content of n-butanol ranges from 0.1 wt % to 1.0 wt %, talkingthe total amount of the solvent to be 100 wt %.

[0053] Incidentally, until this point, description has been made on theproduction method of TAC film using the polymer solution in whichtriacetyl cellulose is dissolved in the solvent containingdichloromethane as the main component; however, in the production methodof cellulose film of the present invention, the main solvent componentmay be an organic solvent such as lower fatty alcohols, and a chlorideof a lower fatty hydrocarbon other than dichloromethane. Additionally,the solute may be a cellulose ester other than triacetyl cellulose.Furthermore, the added poor solvent is not limited to n-butanol, but itmay be any solvent which is highest in boiling point among the materialscontained in the solvent of the prepared polymer solution, and isinferior in the solubility of cellulose ester to the organic solventwhich is the main component of the solvent.

EXAMPLES

[0054] Description will be made below on the TAC film production byapplying the production method of cellulose film of the presentinvention, and the performed measurement of the residual amounts of theorganic solvents, together with the comparative examples.

[0055] At the beginning, example 1 produced the TAC film by using theproduction line 1 shown in FIG. 1. In the preparation of the polymersolution, the triacetyl cellulose synthesized from cotton linter was notmixed; the triacetyl cellulose synthesized from wood pulp (20 parts byweight), a plasticizer (2.2 parts by weight), and an ultraviolet lightabsorber (0.02 parts by weight) were used; and the solvent was preparedso as to give the composition ratios specified below, for which thepolymer solution prepared by the polymer solution preparation apparatus100 was taken to be 100 wt %. Additionally, in the swelling device 600,a solvent gas composed of nitrogen gas and added methanol(methanol:nitrogen=2:8) was sprayed onto the filmy object dischargedfrom the soft film drying zone 11, thereby swelling once the filmyobject.

Example 1

[0056] Dichloromethane: 79.6 wt %

[0057] Methanol: 19.9 wt %

[0058] n-Butanol: 0.5 wt %

[0059] Additionally, in examples 2 to 4, the TAC films were producedunder the same conditions as those in example 1, except that theconditions under which the filmy object discharged from the soft filmdrying zone 1 was once swollen, was changed to each condition specifiedbelow. In other words, the composition ratios of the polymer solutionswere the same as those in example 1.

Example 2

[0060] Application of a solvent (methanol:water=1:1) in 0.5 cc/m².

Example 3

[0061] Spray of Water vapor at 120° C.

Example 4

[0062] Rinsing with water by watersoaking.

[0063] Furthermore, in example 5, the TAC film was produced under thesame conditions (the composition ratios of the polymer solution, etc.)as those in example 1, except that the filmy object discharged from thesoft film drying zone 11 was not once swollen.

Example 5

[0064] Between the soft film drying zone 11 and the late stage dryingzone 12 shown in FIG. 1, the filmy object was not once swollen, and thefilmy object discharged from the soft film drying zone 11 was deliveredto the late stage drying zone 12, thereby performing the continuousdrying.

[0065] Additionally, furthermore, in respective examples 6 and 7 andcomparative examples 1 and 2, the TAC films were produced under the sameconditions as those in example 5, except that the solvent compositionratios of the polymer solution were changed as the respective conditionsspecified below. In other words, in the same manner as that in Example5, the filmy object discharged from the soft film drying zone 11 was notonce swollen.

Example 6

[0066] Dichloromethane: 99.0 wt %

[0067] Methanol: 0.9 wt %

[0068] n-Butanol: 0.1 wt %

Example 7

[0069] Dichloromethane: 70.0 wt %

[0070] Methanol: 29.0 wt %

[0071] n-Butanol: 1.0 wt %

Comparative Example 1

[0072] Dichloromethane: 79.6 wt %

[0073] Methanol: 20.31 wt %

[0074] n-Butanol: 0.09 wt %

Comparative Example 2

[0075] Dichloromethane: 79.6 wt %

[0076] Methanol: 19.29 wt %

[0077] n-Butanol: 1.01 wt %

[0078] On the TAC films produced in respective examples 1 to 7 andcomparative examples 1 and 2, described above, the total residual amountof the organic solvent, the residual amount of dichloromethane, and theresidual amount of n-butanol were respectively measured by gaschromatography, and the results as shown in Table 1 were obtained. Table11 Total Residual Residual residual amount of amount amountdichloromethane of n-butanol (wt %) (wt %) (wt %) Example 1 0.34 0.030.31 [0.5 Wt %, Solvent gas] Example 2 0.34 0.03 0.31 [0.5 Wt %, Solventapplication] Example 3 0.35 0.04 0.31 [0.5 Wt %, Water vapor] Example 40.35 0.04 0.31 [0.5 Wt %, Water soaking] Example 5 0.37 0.05 0.32 [0.5Wt %, No swelling] Example 6 0.18 0.09 0.09 [0.1 Wt %, No swelling]Example 7 0.48 0.04 0.44 [1.0 Wt %, No swelling] Comparative Example 10.45 0.11 0.34 [0.09 Wt %, No swelling] Comparative Example 2 0.51 0.030.48 [1.01 Wt %, No swelling]

[0079] Table 1 shows the total residual amount (wt %) of the organicsolvent, the residual amount of dichloromethane (wt %), and the residualamount of n-butanol (wt %), in a single horizontal row, for each exampleor each comparative example. These three residual amounts are theresidual amounts in the TAC film immediately after having beendischarged from the late stage drying zone 12.

[0080] The present inventors discovered, as a result of diligentresearch, that in order to substantially reduce the effects on theenvironment ascribable to the solvent remaining in the TAC film afterproduction, in the form of the finished TAC film product, the residualamount of dichloromethane is required to be 0.1 wt % or less, andadditionally the total residual amount of the organic solvent isrequired to be 0.5 wt % or less. From the results shown in Table 1, forthe TAC film produced in any of examples 1 to 7, the residual amount ofdichloromethane is 0.1 wt % or less, and additionally the total residualamount of the organic solvent is 0.5 wt % or less. Accordingly, in theTAC film produced in any of examples, the effects of the residualsolvent on the environment can substantially be suppressed. However, inthe TAC film produced in comparative example 1, wherein the content ofn-butanol is 0.09 wt %, the total residual amount of the organic solventis 0.5 wt % or less, but the residual amount of dichloromethane takes aslightly higher value of 0.11 wt %. On the contrary to comparativeexample 1, in the TAC film produced in comparative example 2, whereinthe content of n-butanol is 1.01 wt %, the residual amount ofdichloromethane is 0.1 wt % or less, but the total residual amount ofthe organic solvent takes a slightly higher value of 0.51 wt %. As canbe seen from these results, in order to produce the cellulose film whichsubstantially vanishes the effects of the residual solvent on theenvironment, n-butanol has only to be added in the content range from0.10 wt % to 1.00 wt % in the preparation process of the polymersolution, taking the total amount of the solvent in the prepared polymersolution to be 100 wt %. Turning to a comparison of example 1 withexample 5, both examples being the same in the addition amount ofn-butanol, the residual amount of dichloromethane remaining in the TACfilm is larger in example 5 than in example 1. Such a matter is also thecase in comparison of any example of examples 2 to 4 with example 5. Ascan be seen from these results, by swelling once the filmy objectbetween the soft film drying zone 11 and the late stage drying zone 12,the evaporation of dichloromethane in the late stage drying zone 12 ispromoted. A comparison of examples 1 and 2 with examples 3 and 4indicates that the evaporation of dichloromethane is more promoted byswelling the filmy object with a solvent than with water. Incidentally,the peeling off operation with the peeling roll was able to be morerapidly performed in the production of the TAC film in any example thanin the production of the TAC film in any comparative example.

What is claimed is:
 1. A production method of cellulose film whichproduces cellulose film by preparing a polymer solution throughdissolving cellulose ester in a solvent containing a prescribed organicsolvent as the main component, forming a filmy object from the preparedpolymer solution, and evaporating the solvent remaining in the filmyobject; and wherein: the polymer solution is prepared by adding a poorsolvent, which is highest in boiling point among the materials containedin the solvent, in the content of 0.1 to 1.0 wt %, where the totalsolvent amount contained in the prepared polymer solution is taken to be100 wt %; and the solubility of cellulose ester in the poor solvent isinferior to the solubility of cellulose ester in the organic solventwhich is the main component of the solvent.
 2. The production method ofcellulose film according to claim 1 wherein the organic solvent as themain component is dichloromethane, and the polymer solution is preparedby adding an alcohol having 1 to 2 carbon atoms in addition to the poorsolvent.
 3. The production method of cellulose film according to claim 2wherein the polymer solution is prepared by adding dichloromethane in acontent of 70 to 99 wt %, and simultaneously an alcohol having 1 to 2carbon atoms in a content of 0.9 to 29.0 wt % where the total amount ofthe solvent in the prepared polymer solution is taken as 100 wt %. 4.The production method of cellulose film according to claim 1 wherein thepoor solvent is an alcohol having the boiling point falling in the rangefrom 80 to 170° C.
 5. The production method of cellulose film accordingto claim 1 wherein, in the case where the poor solvent is mixed in anin-line mode, a static mixer arranged in the piping is used for addingand mixing the poor solvent.
 6. The production method of cellulose filmaccording to claim 1 wherein the solid content of the polymer solutionfalls in the range from 15 to 30 wt %.
 7. The production method ofcellulose film according to claim 1 wherein a material containing as themain component thereof the cellulose acetate synthesized from wood pulpis used as the cellulose ester.
 8. The production method of cellulosefilm according to claim 1 wherein at least two or more kinds of polymersolutions of cellulose ester are simultaneously or successivelysubjected to flow casting.
 9. The production method of cellulose filmaccording to claim 8 wherein in the process of forming film by flowcasting of the polymer solutions of cellulose ester, the film is made tobe swollen, on the way of the drying process thereof or after drying,and dried.
 10. The production method of cellulose film according toclaim 8 wherein in the process of forming film by flow casting of thepolymer solutions of cellulose ester, the film is made to be swollen bywatersoaking the film, on the way of the drying process thereof or afterdrying, and dried.
 11. The production method of cellulose film accordingto claim 8 wherein in the process of forming film by flow casting of thepolymer solutions of cellulose ester, the film is made to be swollen byapplying water vapor thereto, on the way of the drying process thereofor after drying, and dried.
 12. The production method of cellulose filmaccording to claim 8 wherein in the process of forming film by flowcasting of the polymer solutions of cellulose ester, the film is made tobe swollen by applying a solvent onto the film, on the way of the dryingprocess thereof or after drying, and dried.
 13. The production method ofcellulose film according to claim 8 wherein in the process of formingfilm by flow casting of the polymer solutions of cellulose ester, thefilm is made to be swollen by applying a solvent gas thereto, on the wayof the drying process thereof or after drying, and dried.
 14. Acellulose film wherein, in the form of the finished film product, theresidual amount of dichloromethane is 0.1 wt % or less, and additionallythe total residual amount of the solvent is 0.5 wt % or less.
 15. Aprotective film for polarizing plate produced according to claim
 1. 16.A protective film for polarizing plate produced according to claim 14.17. An optical functional film produced according to claim
 1. 18. Anoptical functional film produced according to claim
 14. 19. Anpolarizing plate and a liquid crystal display produced according toclaim
 1. 20. An polarizing plate and a liquid crystal display producedaccording to claim 14.